German patent document DE 10 2006 013 166 discusses a method and a control device for determining an opening voltage of an injector having a piezo actuator. The opening of the injector is determined by interrupting the current supply at a holding voltage and then measuring a voltage change present at the piezo actuator and comparing it with a comparison voltage increase.
A fuel injector, e.g., a common rail injector (CRI), frequently has at least one actuator. The at least one actuator may be a piezo actuator, for instance. In this case, a control voltage is applied to the at least one actuator in order to inject fuel into an internal combustion engine. The at least one actuator then experiences an expansion because of the applied control voltage, and it simultaneously exerts pressure on a switching chain disposed on the at least one actuator, which has an hydraulic coupler, for example. A switching valve situated behind the switching chain, possibly a hydraulic servo valve, is moved into an opening position in this manner. If the switching valve is in its opening position, then the fuel is able to flow from the fuel tank into the internal combustion engine.
For precise metering of the fuel injected into the internal combustion engine, the control voltage should be selected in such a way that the switching valve is shifted into its opening position using a defined excess of force. The fact that the control voltage for shifting the switching valve into its opening position is a function of a rail pressure must be taken into account here.
The control voltage is frequently determined individually for each fuel injector during the injector production. A characteristics curve for the control voltage determined for the opening of the fuel injector as a function of the particular rail pressure is subsequently stored in an injection control device for the control of the fuel injector. This is also referred to as injector-specific voltage balancing (ISB).
However, both the at least one actuator and also the switching chain are subject to drift during their service life. Thus, during operation of the fuel injector in a vehicle, there also results a deviation between the control voltage required to open the fuel injector and the original characteristics curve.
It is therefore advantageous if the characteristics curve of the control voltage required for opening the fuel injector is checked during operation of the fuel injector. This check should be carried out especially by taking the particular rail pressure into account. However, the prior art currently provides no possibilities for performing this check without a loss in comfort for the driver, for instance as a result of a noise generation, or without noticeable effect on the dynamic response of the vehicle.
The present invention provides a method having the features described herein and an evaluation device having the features described herein.
The method according to the present invention is based on the understanding that the frequency and the volume of the noise generated during an assessment of the method of functioning of the fuel injector in response to the application of a control voltage is able to be reduced considerably if the particular control voltage for a no-torque-generating injection is applied to the at least one actuator of the fuel injector. Such a no-torque-generating injection is frequently also referred to as post-injection.
In contrast, in the method according to the present invention, there is hardly any noise generation in a performed no-torque-generating injection. In the same way, the dynamic behavior of the vehicle when the method according to the present invention is carried out while driving is hardly affected. As a result, the method according to the present invention is able to be performed regularly during driving without resulting in a loss of comfort for the driver.
Moreover, the method according to the present invention is based on the understanding that a completed no-torque-generating injection is able to be verified by a sensor device for determining a fuel content in the exhaust tract, which is usually installed in the vehicle already. To execute the method, it will therefore not be necessary to install an additional sensor device in the internal combustion engine or in the exhaust tract. This saves the expense and installation space for an additional sensor device. The measured fuel content may be a fuel/air ratio or a fuel quantity in the exhaust tract. The no-torque-generating injection may thus be verified even after the fuel in the engine has been ignited.
It may be determined that the application of the control voltage to the at least one actuator does not cause the fuel injector to open if the determined fuel content is less than the comparison value, and that the application of the control voltage to the at least one actuator causes the fuel injector to open if the determined fuel content is greater than the comparison value. In this way it is easy to determine whether drift of the at least one actuator or the switching chain at the fuel injector is present.
In one specific embodiment of the method, the fuel content in the exhaust tract is determined with the aid of a Lambda sensor. Virtually all vehicles with a weight of more than 1,500 kgs are normally equipped with a Lambda sensor, which is provided to ensure that the specified emission values will not be exceeded. Such a Lambda sensor permits a reliable verification of a performed no-torque-generating injection.
The control voltage may be applied at the at least one actuator at a piston position at which no torque is released during the combustion of fuel possibly injected into the engine. This makes it virtually impossible for noise to be generated when executing the method.
In one specific embodiment, the application of the control voltage to the at least one actuator is executed at a piston position at which the fuel possibly injected into the engine is no longer ignited. This prevents the generation of combustion noise or heat when implementing the method. This specific development of the method is therefore especially suitable for an implementation while driving.
In addition, a rail pressure may be measured and the method of functioning of the fuel injector evaluated as a function of the rail pressure. The method thus also considers the relation between the control voltage required to open the fuel injector and the instantaneously present rail pressure.
In one further development of the method, an initial value for the control voltage is specified at the beginning of the method. For example, such an initial value may be the control voltage specified by the manufacturer for the newly produced fuel injector at the current rail pressure.
In one further development, if the determined fuel content is smaller than the comparison value, then the control voltage is increased by a first specified voltage difference and the method repeated at least once more in order to determine as newly determined control voltage a control voltage whose application to the at least one actuator causes an opening of the fuel injector. Therefore, the method may not only be used to determine whether drift has occurred at the at least one actuator or the switching chain of the fuel injector. In addition, a control voltage suitable for opening the fuel injector may be determined anew as soon as such a situation is determined.
In the same way, if the determined fuel content is greater than the comparison value, then the control voltage may be reduced by a second specified voltage difference, the method repeated at least once more, and then the smallest control voltage be determined as newly ascertained control voltage, at whose application to the at least one actuator an opening of the fuel injector is determined. The second voltage difference may be selected substantially lower than the first voltage difference. This makes it possible to determine the smallest control voltage that induces the fuel injector to open in a relatively precise manner. Following a longer operation of a fuel injector, the present invention therefore makes it possible to newly determine the control voltage to be applied to the actuator for a reliable injection with a minimum tolerance range during a normal driving operation. The control voltage to be applied to the particular actuator may be determined anew as a function of the rail pressure. The determined correction values may then be individually stored in a correction characteristics map.
In one further development of the method, a characteristics curve for an optimal control voltage is determined as a function of the rail pressure on the basis of the newly determined control voltage. In the application of the fuel injector, a number of empty points are specified that permit a sufficiently unambiguous identification of the actuator. When corresponding releases exist, which are, for example, a rail pressure, an exhaust-gas temperature or further stationarities, then the empty points with their control voltage variations are measured one after the other in order to check the installed actuators. If all empty points of the actuators have been measured, then the fingerprint created in this manner is compared with the minimum voltage requirement values originally stored for this actuator and stored in the characteristics map. These characteristics map values are then available to correct the charge voltage control of the individual actuators.
The aforementioned advantages of the method according to the present invention also apply to a corresponding evaluation device. This evaluation device may also be used for a drift correction of the actuators installed on a fuel injector, thereby ensuring perfect functioning of the actuators during the entire operation.
Additional possible uses and advantages of the present invention result from the following descriptions of exemplary embodiments, which are shown in the figures.